Dual-input 18-pulse autotransformer rectifier unit for an aircraft ac-dc converter

ABSTRACT

A dual-input 18-pulse autotransformer rectifier unit for more electric aircraft AC-DC converter uses an autotransformer with a nine-phase output to condition AC power prior to DC rectifying the AC power.

TECHNICAL FIELD

The present disclosure is related generally to aircraft electric powersystems, and particularly to a light weight dual-input nine-phaseeighteen-pulse Autotransformer Rectifier Unit for a More ElectricAircraft AC-DC Converter.

BACKGROUND OF THE INVENTION

Modern aircraft include generators that generate power during flight andprovide the generated power to onboard aircraft electric power systems.The generators utilize rotation of the aircraft engine to generate ACpower using known power generation techniques. Power generated in thismanner is typically 230V 400 Hz AC power. While the aircraft is onground, aircraft engines can be turned off, the onboard generator ceasesgenerating power, and the onboard electric system instead receives ACpower from a ground cart. Power provided from the ground cart istypically 115V 400 Hz AC power.

While the power sources provide AC power, aircraft components oftenrequire DC power instead of AC power. AC-DC power conversion may beaccomplished with a plurality of diode pairs, where each pair isconnected to a different phase of the AC input, to provide a rectifiedDC output. However, this type of AC-DC conversion leads to substantialcurrent harmonics that pollute the electric power generation anddistribution system. To reduce current harmonics, multi-phaseautotransformers are employed to increase the number of AC phasessupplied to the rectifier unit. For example, in an 18-pulse passiveAC-DC converter the autotransformer is used to transform the three-phaseAC input, whose phases are spaced at 120°, into a system with ninephases spaced at 40°. This has the effect of reducing the harmonicsassociated with the AC-DC conversion.

SUMMARY OF THE INVENTION

Disclosed is a passive AC-DC converter having an autotransformer. Theautotransformer has a plurality of first high voltage AC inputs, aplurality of second low voltage AC inputs, a winding topology having aplurality of windings corresponding to each of multiple phases, theplurality of windings configured such that the autotransformer generatesan 18-pulse AC input current waveform, and a set of autotransformeroutputs. The passive AC-DC converter further includes a bridge rectifierconnected to the set of autotransformer outputs and a DC output from thebridge rectifier.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a nine-phase Autotransformer based AC-DC Converter inan aircraft electric power system context.

FIG. 2 illustrates dual-input nine-phase 40° phase shift autotransformerconnected to a nine-phase rectifier unit.

FIG. 3 illustrates autotransformer core and winding configuration.

FIG. 4A illustrates 21-vector diagram representing the physical windingsof a nine-phase autotransformer.

FIG. 4B illustrates the 21-vector diagram of FIG. 4A with the addedillustration of resultant vectors.

DETAILED DESCRIPTION

FIG. 1 illustrates a power conversion system 110 for use in an aircraftelectric power system. The power conversion system 110 includes apassive AC-DC converter 120 that accepts either a three-phase 230V ACinput 140 from a 230V aircraft power source or a three-phase 115V ACinput 150 from a 115V ground-cart power source and provides power to aDC load 165 via a DC+ connection 180, a DC− connection 182, and a 0Vconnection 181. The passive AC-DC converter 120 includes anautotransformer 160 that accepts three phase AC inputs 162, 164 andprovides a nine phase AC output 163 to a rectifier 170. The DC outputs180, 181, 182 are connected to the rectifier 170. The particular powersource 140, 150 connected to the passive AC-DC converter 120 iscontrolled by a set of break-before-make switches 130 on each input line162, 164. Each input 162, 164 is also filtered by a corresponding ACfilter 190, 192.

FIG. 2 illustrates an example passive AC-DC converter 200 that can beused as the AC-DC converter 120 of FIG. 1. Within the passive AC-DCconverter 200, an autotransformer 260 accepts either the three-phasevoltage input 262, or the three-phase voltage input 264. The passiveAC-DC converter 200 converts either input 262 or 264 to a nine-phase ACoutput 263. The nine-phase AC output 263 is rectified to DC voltage by arectifier 270. The rectifier 270 provides a DC+ output 280 and a DC−output 282. The DC+ and DC− outputs 280, 282 provide power to a powerbus that distributes DC power to onboard aircraft components. Theautotransformer 260 nine-phase output currents are square-wave, and thethree-phase input current is an 18-step waveform closely approximating adesired sine-wave.

In order to utilize a single passive AC-DC converter 200 for both the230V AC power input 262 and the 115V AC power input 264 and maintain aconstant DC output voltage, the autotransformer 260 further includes a115V input to 230V step up when a 115V input 264 is connected. This stepup ensures that a constant DC voltage output is provided from the DCoutputs 280, 282 regardless of whether 115V AC source 264 or 230V ACsource 262 is connected. 230V AC voltage is then stepped down by a ratioof γ, where 1≧γ≧0.5. DC output voltage 265 is proportional to step downratio γ, and a designer can select step down ratio γ to meet aparticular DC output voltage requirement. Thus, the autotransformer 260nine-phase output 263 is γ×230V for either a 230V AC input 140 or a 115VAC input 150. Since the nine-phase output 163, 263 is independent of theparticular selected AC input 140, 150 (illustrated in FIG. 1), therectifier output 280 and 282 is also independent of the particularselected inputs 140, 150. Furthermore, the kVA rating of theautotransformer 160, 200 is

$\frac{46.7\%}{\gamma},$

where γ is the autotransformer step down ratio and where 1≧γ≧0.5.

Further referring to FIG. 2, the autotransformer AC output currentwaveform at pin 1 is pulsed, with high harmonic current contents. Theoutput at pin 2 is also pulsed with a 40° phase shift relative to theoutput of pin 1. The same applies to outputs of pins 3, 4, 5, 6, 7, 8,and 9. The autotransformer 260 synthesizes the nine output pulsedcurrent waveforms into an 18-step current waveform that closelyapproximates a desired sinewave current at 230V AC input 262 A, B and C,and similarly at 115V AC input 264 a, b and c. Undesirable harmoniccurrents at nine-phase outputs 263 are cancelled through theautotransformer 260.

FIG. 3 schematically illustrates the autotransformer 260. Theautotransformer 260 has a three-legged core 310, with physical windingsA0-A6, B0-B6, and C0-C6 wound respectively about the three core legs,320A, 320B, and 320C. The 230V 3-phase AC inputs 262A, B, C are locatedon each phase leg, as well as the 115V 3-phase AC inputs 264 a, b, c.Outputs 1-9 output nine-phase power from the autotransformer 260.

With continued reference to FIGS. 2 and 3, FIG. 4A illustrates a vectordiagram 400 of the physical windings A0-A6, B0-B6, C0-C6 of an exampleautotransformer 260 from the example AC-DC converter illustrated in FIG.2 with step down ratio γ=0.9. FIG. 4B illustrates the vector diagram ofFIG. 4A with the added illustration of resultant vectors. The arrowlength of each winding A0-A6, B0-B6, C0-C6 is proportional to the numberof winding turns, and the arrow points from the start of the winding tothe finish of the same winding. The three 230V AC input voltage vectorsare represented by drawing straight lines from A, B and C to trianglecenter o, to form A-o, B-o, and C-o. The three 115V AC input voltagevectors are represented by drawing straight lines from a, b and c totriangle center o, to form a-o, b-o, and c-o. Nine output voltagevectors are represented by drawing straight lines from points 1 through9 to triangle center o, to form nine output voltage vectors 1-o, 2-o,3-o, 4-o, 5-o, 6-o, 7-o, 8-o, and 9-o. The ratio of the length of vector1-o to the length of vector A-o is the autotransformer output voltagestep down ratio γ from 230V, where γ=0.9. The ratio of the length ofvector A-o to the length of vector a-o is 2, the fixed voltage ratio of230V and 115V AC inputs. The phase angle from 1-o to 2-o is 40°, and theangle from 1-o to 9-o is also 40°. The phase angle from 4-o to 3-o is40°, and the phase angle from 4-o to 5-o is 40°. Similarly, the phaseangle from 7-o to 6-o is 40°, and the phase angle from 7-o to 8-o is40°. As illustrated, all winding segments A0-A6, B0-B6, and C0-C6 drawnas parallel to each other are on a shared phase leg of theautotransformer core 310 in FIG. 3. The phase legs 320A, B, C of thethree legged autotransformer core 310 receive windings A0-A6, B0-B6, andC0-C6 respectively.

The nine AC power output connections 1, 2, 3, 4, 5, 6, 7, 8, and 9 areillustrated as solid circles, and provide a nine-phase output to therectifier 270, illustrated in FIG. 2. The 230V AC inputs 262 illustratedin FIG. 2 are connected to the autotransformer 260 at connection pointsA, B, and C. The 115V AC inputs 264 of FIG. 2 are connected to theautotransformer 260 at connection points a, b, and c. Connection pointsa, b, and c connect the 115V input to the autotransformer 260, such thatthe autotransformer 260 steps up the AC input 264 to an equivalent 230VAC before the desired step down, γ, resulting in a consistent nine-phaseAC output voltage regardless of which of the two inputs 262, 264 isconnected to its respective 230V AC or 115V AC source.

In addition to the winding polarity of the winding segments A0-A6,B0-B6, C0-C6, the number of winding turns of A0-A6, B0-B6 and C0-C6 isnormalized to that of A0, B0, C0. It should be understood that the turnsratio between the various winding segments A0-A6, B0-B6, C0-C6 isdetermined by the step down ratio γ. In the illustrated example of FIGS.4A and 4B, the autotransformer 260 steps down the 230V AC input, suchthat the AC output of the autotransformer 260 is γ=0.9 of the 230V ACinput. To achieve this step down, the normalized number of turns are: A0is 1 turn, A1 is 1.690 turns, A2 is 4.668 turns, A3 is 3.926 turns, A4is 3.433 turns, A5 is 3.572 turns, and A6 is 3.177 turns. The number ofturns on each of the winding segments B0-B6 and C0-C6 arecorrespondingly identical to those of A0-A6.

While the particular example winding segments of FIG. 4 are illustratedwith a step down ratio of 0.9 (90%) (the nine AC output voltages are 90%of the AC input voltage at A, B and C), a worker of skill in the artwould be able to adapt the vector diagram disclosed above to achieve adifferent step down ratio using the same principles as 0.9 step downexample. By way of example, a person of skill in the art could utilizethe above teachings to adjust the turns ratios to achieve varied stepdown ratios to achieve 1≧γ≧0.5 for step down application.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this invention. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this invention.

1. A passive AC-DC converter comprising: an autotransformer, whereinsaid autotransformer comprises; a plurality of first high voltage ACinputs; a plurality of second low voltage AC inputs; a winding topologyhaving a plurality of windings corresponding to each of multiple phases,said plurality of windings configured such that said autotransformergenerates an 18-pulse AC input current waveform; and a set ofautotransformer outputs; a bridge rectifier connected to said set ofautotransformer outputs; and a DC output from said bridge rectifier. 2.The passive AC-DC converter of claim 1, wherein said set ofautotransformer outputs comprises a nine phase AC output.
 3. The passiveAC-DC converter of claim 2, wherein said nine-phase AC output isconfigured such that an approximately sinusoidal current is drawnthrough said autotransformer from said plurality of first AC inputs orsaid plurality of second AC inputs.
 4. The passive AC-DC converter ofclaim 2, wherein said autotransformer further comprises a plurality ofwinding segments on each of multiple phase legs of said autotransformer,and wherein each of said plurality of winding segments has a turn rationormalized to a designated winding segment on said phase leg.
 5. Thepassive AC-DC converter of claim 4, wherein each of said phase legscomprises a low voltage input corresponding to one input of saidplurality of second inputs, wherein said low voltage input is connectedto a junction between winding segments on said phase leg.
 6. The passiveAC-DC converter of claim 5, wherein each of said low voltage inputs isconfigured to step up said low voltage input to an expected voltage of acorresponding high voltage input.
 7. The passive AC-DC converter ofclaim 4, wherein said plurality of winding segments on each phase leg ofsaid autotransformer are configured such that each corresponding outputof said nine-phase output is a stepped down voltage relative to anexpected AC voltage of said high voltage input.
 8. The passive AC-DCconverter of claim 7, wherein said stepped down voltage is a percentageof said expected AC voltage of said high voltage input, and wherein saidpercentage is dependent upon said turn ratios of said winding segments.9. The passive AC-DC converter of claim 8, wherein said stepped downvoltage is determined with a step down ratio of 1≧γ≧0.5.
 10. The passiveAC-DC converter of claim 9, wherein a kVA rating of the autotransformeris $\frac{46.7\%}{\gamma}.$
 11. The passive AC-DC converter of claim 9,wherein each phase leg of said autotransformer comprises a first windingsegment having a turn ratio of 1, a second winding segment having a turnratio of 1.690, a third winding segment having a turn ratio of 4.668, afourth winding having a turn ratio of 3.926, a fifth winding segmenthaving a turn ratio of 3.433, a sixth winding segment having a turnratio of 3.572, and a seventh winding segment having a turn ratio of3.177, for a step down ratio of 0.9.
 12. The passive AC-DC converter ofclaim 1, wherein said autotransformer has at most 21 windings, andwherein said windings are configured such that a pulsed output currenton each of nine phase outputs is converted to a near sinusoidal currentwaveform via eighteen steps at AC inputs.
 13. The passive AC-DCconverter of claim 12, wherein said autotransformer comprises threephases, and wherein seven of said at most twenty-one windings are oneach of said phases.
 14. The passive AC-DC converter of claim 1, furthercomprising: a set of first switches connecting said plurality of firstinputs to said autotransformer; a set of second switches connected saidplurality of second inputs to said autotransformer; a controllercontrollably coupled to said first and second set of switches, whereinsaid controller is operable to prevent one of said first inputs and saidsecond inputs from connecting to said autotransformer when the other ofsaid first inputs and said second inputs is connected to saidautotransformer.
 15. The passive AC-DC converter of claim 14, whereineach of said switches in said set of first switches is abreak-before-make switch, and wherein each of said switches in said setof second switches is a break-before-make switch.